The present invention is generally related to a method and apparatus for measuring warpage in a specimen and, more particularly, is related to a method and apparatus using a projection moirxc3xa9 technique to measure warpage in a temperature controlled environment which simulates on-line manufacturing processes or facilitates laboratory development processes.
The surface flatness of objects, such as printed circuit boards, integrated circuit (IC) packages, ceramic and metal substrates, papers, plastics, woven items and the like, can be very important and is often of special concern. For example, the manufacture of printed circuit boards, IC packages and other electronic interconnection products is a multi-billion dollar global industry, and the flatness of these products is critical to their ability to undergo further manufacturing steps and to their ultimate reliability in operation as parts of computer, automobile, telecommunications, aerospace, military and other electronic systems. Non-flatness, or warpage, is a frequent problem in manufacturing due to inadequacies in design, materials, and/or processing of components, which are typically complex devices composed of several different materials. The ability to analyze surface flatness, especially changes in surface flatness (warpage) associated with changing temperatures, plays an important role in the designing, manufacture, processing and maintaining of objects for which surface flatness is an important characteristic.
One method of analyzing surface flatness is projection moirxc3xa9 interferometry. Projection moirxc3xa9 is a full-field, noncontact method of measuring out-of-plane displacements for in-plane deformations of a structure. A typical projection moirxc3xa9 fringe pattern is a series of light and dark fringe lines of equal change in surface position which map out the contour change of an object much the same way a topography map delineates the contour of land. FIG. 1 illustrates a simplified projection moirxc3xa9 system 20. Generally, projection moirxc3xa9 interferometry fringe lines are formed by a plurality of phase shifted laser beams 22 projected onto a workpiece 24. A laser 26 generates a laser beam 28 which is projected into a shearing interferometer 30. The laser beam 28 is expanded by beam expander 32 and transmitted to beam splitter 34. The shearing interferometer 30 is thus able to generate a plurality of interfering laser beams that form fringe patterns fanning out onto workpiece 24. The spatial frequency of the fringes may be adjustable to a desired horizontal and/or vertical spacing. Some projection moirxc3xa9 systems employ an actuated mirror (not shown) which sweeps the beam 22 across workpiece 24 in a manner which further improves the analysis of the moirxc3xa9 patterns projected onto workpiece 24. A camera 36 detects the moirxc3xa9 fringe patterns and provides the image of the workpiece 24 and the projected fringe patterns to processor 38. The image detected by camera 36 is displayed on the video display screen 40. The operator may interface with and/or control the projection moirxc3xa9 system and camera through an interface, such as keyboard 42.
Shadow moirxc3xa9 fringe analysis is another fringe pattern analysis technique employing a light source projected through a glass plate having overlaying grating lines (reference grating) which are projected onto the workpiece as specimen gratings. The interference of the reference and specimen gratings produce moirxc3xa9 fringes. Like the projection moirxc3xa9 method, a camera captures the fringe image created by the shadow moirxc3xa9 system and a processor processes the images detected by the camera.
Typically, an image of an un-deformed workpiece 24 (FIG. 1) is recorded by the camera 36 and stored within processor 38 for comparison against a deformed workpiece 24 (or the same workpiece 24 after deformation). By comparing the differences in the projected fringe patterns on the un-deformed and the deformed workpiece 24, the nature of any warpage and/or deformations in the workpiece 24 can be analyzed. However, the technique of comparing a deformed workpiece with an un-deformed workpiece (or comparing a change in deformation in the same workpiece) limits the analysis to two static points in time. That is, two views of a workpiece 24 are compared. For example, images of the initial un-deformed conditions and the subsequent deformations of workpiece 24 after a process may be compared.
The deformations experienced by a workpiece 24 as the workpiece is subjected to a process cannot be analyzed with this static projection moirxc3xa9 analysis approach. A dynamic analysis approach would provide a much better overall picture of the deformation process. For example, if the workpiece is a printed circuit board traveling down an assembly line through a series of reflow oven zones wherein various components are soldered to the circuit board, the circuit board would presumably start out in an initial un-deformed state, and then be subjected to a series of heating and cooling cycles which may induce various degrees of warpage at various locations on the circuit board at different times during the process as the electronic components are soldered to the printed circuit board.
Furthermore, tracking a workpiece 24 with a camera 36 and a projection moirxc3xa9 system 20 as the workpiece travels down the manufacturing assembly line presents numerous and nearly insurmountable difficulties. The oven enclosures which solder components to the circuit board provide restricted access to the projection moirxc3xa9 system 20. If a single projection moirxc3xa9 system 20 is used, the laser 26, shearing interferometer 34, and camera 36 need to travel along the same path as the workpiece in a manner such that fringe patterns can be projected on the workpiece 24 and detected by camera 36 during the entire manufacturing process, including the soldering process conducted inside of the oven enclosures. Alternatively, a plurality of projection moirxc3xa9 interferometer systems 20 might be used at pre-selected locations along the manufacturing assembly line, however, this approach would have the higher costs of having multiple lasers 26, shearing interferometers 30 and cameras 36. Also, difficulties will have to be overcome in image synchronization between the plurality of cameras spaced along the assembly manufacturing line. Additionally, any of the projection moirxc3xa9 systems 20 viewing the workpiece 24 in an oven enclosure would have to overcome special design problems associated with operation in the high temperature environment of an oven enclosure. Thus, the above-described prior art projection moirxc3xa9 analysis method is not practically able to dynamically detect and analyze the entire warpage process that a workpiece 24 may be subjected to during a manufacturing process.
One prior art technique employs a shadow moirxc3xa9 analysis system in which a work sample is placed in a heating chamber. The shadow moirxc3xa9 illumination source is directed into the heating chamber and onto the sample. Subsequently, the temperature in the heating chamber. may be adjusted over time to approximate a predefined temperature/time profile. The camera captures the entire deformation process as the workpiece is subjected to changes in temperature during the simulation of the temperature/time profile. The temperature/time profile in the heating chamber can be designed to simulate an actual manufacturing process, or be specified such that a process engineer can conduct laboratory experiments for use in process and manufacturing method design. The use of a heating chamber for housing and heating a sample for analysis by a shadow moirxc3xa9 system is taught in Ume, U.S. Pat. No. 5,601,364, which is incorporated herein by reference. However, the shadow moirxc3xa9 analysis system taught in Ume contains a single fixed heating source and a single variable heating source. It would be desirable to have additional flexibility in the heating sources, a way to circulate air within the chamber, a way(s) to cool the chamber, and the equally accurate projection moirxc3xa9 method.
Under certain conditions, a projection moirxc3xa9 system is known to provide a more accurate analysis method than the shadow moirxc3xa9 methods. Thus, it would be desirable to have an apparatus and system which could dynamically analyze the warpage of a workpiece as the workpiece is subjected to a varying temperature/time profile. Furthermore, it would be advantageous to provide for a variety of heating means and cooling means whereby the predefined temperature/time profile could be more accurately simulated.
One embodiment of the present invention is a projection moirxc3xa9 method for measuring thermally induced warpage in a workpiece. The workpiece is first placed in the chamber. Next, a grating pattern is projected into the chamber and onto the workpiece. The projected grating pattern forms a fringe pattern on the workpiece, while a camera captures the resulting fringe pattern. Subsequently, the temperature in the chamber is adjusted to approximate a predefined temperature/time profile. The camera records a continuous series of images of the fringe patterns formed due to the deformation of the workpiece during the temperature/time profile simulation, while recording the corresponding temperature and time. Finally, the warpage of the workpiece can be determined by analyzing the sequence of images captured during simulation of the predefined temperature/time profile.
One embodiment of the present invention has a chamber for housing a workpiece, a projection moirxc3xa9 system and a temperature control system. The chamber has a glass top for viewing the workpiece which has been placed in the chamber, a plurality of heating sources and at least one cooling source. The temperature control system employs a processor which controls a plurality of heaters, coolers and fans residing in the chamber such that temperature within the chamber can be regulated according to a predefined temperature/time profile. The heating and/or cooling sources may have a variable output temperature.
The present invention can also be conceptualized as providing one or more methods for subjecting a workpiece to changing the temperature and recording warpage of the workpiece using a projection moirxc3xa9 system. In accordance with one method of the invention, the method may be broadly summarized by the following steps: positioning a workpiece in a chamber; projecting a plurality of grating patterns on the workpiece; regulating temperature in the chamber; recording images of the fringe patterns formed on the workpiece; and analyzing changes in the plurality of fringe patterns.
Other systems, methods, features, and advantages of the present invention are or will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.